I estimated my solar fraction using this calculator.
First of all, solar irradiance vs tilt:

I plan on installing two systems, one for hot water, and another one for space heat. The solar hot water system will be set against the garage, on a vertical wall, with some summer shading from the roof overhang. I used the following website to estimate how much shading the overhang will generate:Sustainable Design.
Here is an estimation of the area of the collector that will get shaded, for each month:

Using these values, I can now estimate the solar fraction for a 100sqft collector (second row=consumption, 3rd row=solar fraction, 4rth row=savings):

I also calculated the excess heat provided by the collector, that could be used for space heating. The yearly value of this excess heat is estimated at $32.
Next, solar fraction for space heat, assuming a 300sqft collector (second row=consumption, 3rd row=solar fraction, 4rth row=savings):

The total yearly savings with a 100sqft hot water collector, and a 300sqft space heat collector, will be $776, out of a $1600 bill. Because we get bi-monthly electric bills, here is the savings for each billing period:

It is interesting to see that the Sep-Oct bill is lower than the Jul-Aug bill. Two factors explain that: the tilt, optimized for winter, and the overhang shadow. These are good features that will reduce summer overheating of the collectors.

Finally, i will add the estimated savings from a $8000 heat pump. The savings calculation assumed 73°F winter temperature settings (we are at 68°F), resulting in a $2000 heating bill (our total electric bill is less than $2000, with about 40% for heat, and 15% for hot water). Due to these false assumptions, the savings are inflated, so I applied a correction factor equal to the actual vs estimated heating bill:
Estimated savings from heat pump = $880 / year (assuming $2000 yearly heating bill).
Savings after correction = $352 (assuming 40% of $2000 total electric bill).

The solar system will cost less than $8000 (cost estimated between $4000 and $5000), yet provide twice the savings of the heat pump.
The solar fraction is estimated at 80% of heating bill, and 45% of total electric bill.